The present invention is directed to a method and an apparatus for the heat treatment of a workpiece and in particular, for the heat treatment of a workpiece having a cavity formed therein. More particularly, the present invention is directed to a method and an apparatus for supporting an outer race member of a universal joint during a heat treatment operation.
It is often difficult to obtain a quality hardened part when the part has cavities formed in it, due to the nature of the heat treating operation used to harden the parts. Typically, a part is heat treated by heating the part in a carborizing atmosphere to a predetermined temperature and subsequently quenching the part with a cooling fluid. If the part is positioned so that the cavity formed in the part opens upwardly, the cooling fluid used in the quenching operation fills the cavity and the cooling fluid is carried out of the quench tanks after the quenching operation is complete. Therefore, an additional operation is required to remove the cooling fluid from the part and to replace the cooling fluid in the quench tank. A portion of the cooling fluid is wasted during this additional operation.
Alternatively, when the workpiece is heat treated with the cavity disposed such as to open downwardly, the carborizing atmosphere used during the heating step of the heat treatment operation becomes trapped in the upper portion of the cavity during the quench segment of the heat treatment. The carborizing atmosphere, therefore, prevents the cooling fluid from entering the cavity, preventing the workpiece from being uniformly quenched. In particular, the interior surface of the cavity may not be sufficiently hardened by the heat treatment process.
The above described problem exists for all workpieces having cavities, but is particularly serious when the interior surface of the cavity performs a load bearing function, since insufficient hardening of a load bearing surface may result in premature failure of the part.
There are many universal joints in common commercial use which include an outer race member having a cavity formed therein, and an inner race member disposed within the cavity. Various means, well known in the art, are used for interconnecting the inner race member with the outer race member, such as bearing balls, which transfer torque between the inner and outer race members. The inner surface of the cavity experiences substantial loads when torque is transferred between the inner and outer race members and, therefore, must be adequately hardened so as to provide a long life for the outer race member.
To avoid the above described problems during a heat treatment operation, the method and the apparatus of the prior art used for heat treating the outer race members of the universal joints and similar workpieces involved placing the outer race member or workpiece in a horizontal position such that the cavity opened horizontally. This avoids the entrapment of either the carborizing atmosphere or the oil within the cavity. However, in practice, when an outer race member is heat treated in a horizontal position a substantial amount of material movement takes place within the part. This material movement results in an undesirably high rejection rate of outer race meabers which have already been manufactured and heat treated.
What is needed, therefore, is a method and an apparatus for the even and reliable heat treatment of a workpiece having a cavity and, particularly, of the outer race member of a universal joint.